You are correct that people will more often select “may” over “might” when they are interchangeable.
In this case, however, you are referring to the past, and should therefore use “might”.
The technical details here are that:
“Might” is (among other things) the past tense of “may”
Your example contains both a primary clause (“I was worried”) and
a secondary clause (“you may/might not like your gift”)
- By convention, secondary clauses match the tenses of their
respective primary clauses
Note that, while these are the technical distinctions in play, in practice most English speakers just throw in whichever one they feel like and are understood just fine. You can't go wrong with using “might” for the past and “may” for the present or future, and I do think it's fairly common to associate “might” with possibilities and “may” with permissions, but you're probably not going to have problems either way.